1. Field of the Invention
The present invention relates in general to a hydraulic control system for a continuously variable transmission of a belt-and-pulley type for a motor vehicle, and more particularly to such a hydraulic control system for regulating a first line pressure for a high-pressure side hydraulic cylinder, based on an error or difference between actual and target speed ratios of the transmission.
2. Discussion of the Related Art
A belt-and-pulley type of continuously variable transmission for an automotive vehicle is known. This transmission includes a first shaft, a second shaft, a first variable-diameter pulley provided on the first shaft, a second variable-diameter pulley provided on the second shaft, a transmission belt connecting the first and second pulleys to transmit power from the first shaft to the second shaft a first hydraulic cylinder for changing an effective diameter of the first pulley, and a second hydraulic cylinder for changing an effective diameter of the second pulley. An example of a hydraulic control system for controlling this type of continuously variable transmission is disclosed in Japanese Patent Application No. 61-37571 (filed Feb. 22, 1986) the subject matter of which is disclosed in U.S. patent application, Ser. No. 016,661 filed Feb. 19, 1987, assigned to the assignee of the present application. The hydraulic control system disclosed therein includes (a) a first pressure regulating valve for regulating a pressure of a pressurized fluid delivered from a hydraulic source, and thereby establishing a first line pressure, (b) a shift-control valve for applying the first line pressure to one of the first and second hydraulic cylinders, while simultaneously permitting a flow of the fluid from the other hydraulic cylinder, thereby changing the effective diameters of the first and second pulleys to control a speed ratio of the transmission, and (c) a second pressure regulating valve for regulating the pressure of a discharge flow of the fluid flowing from the above-indicated other hydraulic cylinder through the shift-control valve, and thereby establishing a second line pressure which is lower than the first line pressure. The shift-control valve is controlled so that the actual speed ratio of the transmission coincides with a target or desired speed ratio.
In the continuously variable transmissions as discussed above, the first and second line pressures are regulated by the respective first and second pressure regulating valves. More specifically, the first pressure regulating valve is controlled so as to maintain the first line pressure at a lowest level required to permit the drive-side pulley to provide a thrust necessary to establish the target speed ratio while minimizing a power loss of the system, while the transmission is operating with its speed ratio kept at a substantially constant level. On the other hand, while the transmission is being shifted at a relatively high rate, the first line pressure is regulated so as to enable the transmission to have a relatively high operating response, i.e., a sufficiently high rate of change of the speed ratio. In the meantime, the second pressure regulating valve is controlled so as to maintain the second line pressure at a lowest level required to enable the transmission to operate without a slip of the belt on the pulleys. Thus, the first and second line pressures are regulated so that the transmission exhibits a sufficiently high response during a shifting operation, while minimizing the power loss in the transmission system of the vehicle.